Student No…………………………………………. Assignment 2 School of Chemical and Metallurgical Engineering University of the Witwatersrand, Johannesburg (Marks are in square brackets) Question 1 [6 marks] For each of the following systems, give a typical example of an application of mass transfer with chemical reaction: (1) gas-liquid systems, (2) liquid-liquid systems, and (3) gas-liquid-solid systems Question 2 [6 marks] Many theories such as the boundary layer, film-penetration, surface-stretch and others are available to explain the behaviour of mass transfer coefficients. The theories are based on both molecular and turbulent diffusion. Explain and/or describe the three mentioned theories. Question 3 [12 marks] Industrial processes in chemical engineering or laboratory operations in which heat or mass or both are transferred from one fluid to another take place on batch and continuous basis. However, due to various demands in industries most of the operations are carried out on a continuous basis. Such continuous operations are carried out on counter-current and co-current mode. In these operations, the concentration of each phase changes with position. (a) With the aid of well labelled diagrams, derive the general equations of the operating lines in counter-current and co-current processes for gas absorption. (b) Illustrate graphically the equilibrium and operating lines for counter-current and cocurrent processes when the solute moves from the absorbing solvent into the carrier gas. Note: State all the assumptions and clearly define all nomenclature used. Question 4 [20 marks] An air-ammonia mixture is being treated with a water stream in a 10 ft pilot scale absorber in a lab test under atmospheric conditions (70°F and 1 atm). The absorber was set up in such a way so as to measure the partial pressure of each component at various locations in the column. It was found that at a distance 5 ft up the column the liquid contained 0.4 wt% ammonia while the partial pressure of ammonia was 10 mm Hg. The solubility data for this system is presented in Table 1. 1 (i) Estimate the interfacial gas and liquid phase concentrations, if, based on previous experiments, the ratio of the liquid mass transfer coefficient (-kL/kG) was found to found to be -1.0 atm/lbmol.ft3. (ii) If the mass transfer coefficient is 12.5 lbmol/h.ft2.atm, estimate the molar flux of ammonia Data: The density of pure water is 62.4 lb/ft3. Take the atomic masses of hydrogen and nitrogen atoms to be 1.0 and 14.0, respectively. Table 1: Ammonia data NH3 partial pressure, atm 0.0045 0.0097 0.0119 0.0158 0.0201 0.0255 0.0309 Liquid concentration, lbmol NH3/ft3 0.0180 0.0367 0.0440 0.0586 0.0733 0.0916 0.1100 Submission Date/Time: 9 March 2020 at 08h00 in the CHMT 3005 hand-in box on the second floor of Richard Ward building! (WRITE ANSWERS BELOW) ------------------------------------------------------------------------------------------------------- 2
